Since switched mode AC/DC converters do not draw only real power uniformly from the power supply, there is a tendency for them to introduce harmonic distortion into the mains supply. Because some of the power drawn is so-called imaginary power, in other words the voltage and current are in quadrature, the power factor of AC/DC converters is less than unity. For high power converters the distortion on the mains can be significant, and this can be detrimental to the supply network; as a result in some jurisdictions regulation has been introduced, such as EN 61000-3-2 in Europe, to limit the mains distortion.
In order to ensure a high power factor, and to comply with such regulation, it is increasingly common to integrate a power factor correction stage within an AC/DC converter. However, since the distorting effect of non-unity power factor is lower for lower drawn mode, the PFC may not be required for low loads, and it is thus known to disable the PFC stage for low load operation. An example of a controller for an AC/DC converter with a switchable PFC stage (that is to say, a PFC stage which may be enabled or disabled) is TEA175x range of products available from NXP Semiconductors®. The enabling and disabling of the PFC stage, in known controllers, is determined by the status of the output power of the controller, which is monitored by means of the power control pin of the controller. The power control pin indirectly senses the output power provided by the converter, typically by means of an isolating opto-coupler which senses the output power (at the secondary side) and passes this information through the isolation of the opto-coupler to the primary side specifically to a control pin of the controller.
However, this arrangement is not ideal because the voltage on the power control pin tends to be noisy and is relatively unstable, which may result in inappropriate enabling or disabling of the PFC stage in order to accommodate this, switching thresholds for the PFC stage, at which the PFC is enabled or disabled, may be set to non-ideal values.